1. Field of the Invention
The present invention relates to a mass spectrometer equipped with a MALDI (matrix-assisted laser desorption/ionization) ion source used when biopolymers and other samples are analyzed qualitatively or quantitatively.
2. Description of Related Art
A MALDI process is a method of vaporizing or ionizing a sample by mixing the sample into a matrix (liquid, crystalline compound, or metal powder) having an absorption band in the wavelength of the used laser light, dissolving and solidifying the matrix, and irradiating the matrix with the laser light. In a laser ionization method typified by a MALDI process, the initial energies during ion creation are distributed in a wide range. Accordingly, in order to converge the distribution in terms of time, delay extraction is employed in most cases. In this method, a pulser voltage is applied with a delay of hundreds of nanoseconds after laser irradiation.
The general concept of delay extraction applied to a MALDI ion source is shown in FIG. 4. An analyte is mixed into a matrix (liquid, crystalline compound, or metal powder), and the mixture is dissolved and solidified, thus preparing a sample 12. The sample 12 is placed on a sample plate 11. To permit the state of the sample 12 to be observed, a lens 16, a mirror 15, and a CCD camera 17 are arranged. Laser light is directed at the sample through another lens 13 and another mirror 14, thus vaporizing or ionizing the sample. The created ions are accelerated by voltages applied to an intermediate electrode 18 and to a base electrode 19, respectively. The accelerated ions are introduced into a TOF (time-of-flight) mass analyzer (not shown).
A sequence of operations for measuring the flight time in the delay extraction is also shown in FIG. 4. First, the intermediate electrode 18 and sample plate 11 are placed at the same potential of Vs. Then, the potential Vs at the intermediate electrode 18 is varied to potential V1 at high speed with a delay of hundreds of nanoseconds since reception of a signal from a laser (not shown) that indicates laser oscillation. Consequently, a potential gradient is created between the sample plate 11 and the intermediate electrode 18 to accelerate the created ions. The time at which the flight time is started to be measured is synchronized with the time of start of variation of the voltage applied to the intermediate electrode.
Because the flight time of the ions is determined by the strength of the accelerating electric field produced between the sample plate 11 and the intermediate electrode 18, the distance between the sample plate 11 and the intermediate electrode 18 must be exactly the same at all times irrespective of the position on the sample plate 11.
In a mass spectrometer using a MALDI process in the ion source, the sample plate 11 is generally made of a conductive material, such as stainless steel, in order to produce an electric field that extracts ions created by laser irradiation. Marks are engraved on the surface of the conductive sample plate 11 to indicate the positions into which the sample is dripped. The number of the engraved marks is generally 96 (12 rows×8 columns), 384 (24 rows×16 columns), or 1536 (48 rows×32 columns).
In use, the sample plate 11 having the engraved marks into which the sample 12 has been dripped is introduced into the mass analyzer. The droplets of the sample 12 are irradiated with laser light while observing the surface of the sample plate 11 with the CCD camera 17. Thus, the sample is ionized. Consequently, a mass analysis is performed. See, JP-A-2003-43014, JP-T-2003-534634, JP-A-2004-347524, and JP-T-2005-513490.
However, where a measurement is performed using plural sample plates each having many engraved round marks for sample, some problems may take place. In particular, it is not known what of the samples is being measured. It is not known what sample is being dripped onto what sample plate. It is impossible to find the relationship between the used sample plate and the data obtained by the measurement.
Furthermore, where sample plates are stocked under the condition in which a sample has been dripped, it is necessary to manage the sample plates individually in order to prevent the measurer from using an erroneous sample plate.
In addition, the surface of each sample plate is uneven and produces distortion. The distortion is on the order of ±0.1 mm but may affect the mass resolution and the mass axis, making it impossible to obtain correct mass spectra. Consequently, it is necessary to make a mass calibration based on information about the distortion.